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Towards a unified model for granite genesis

By
B. W. Chappell
B. W. Chappell
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Published:
January 01, 2004

Most granites result from partial melting within the crust. Granite melts produced at the lowest temperatures of partial melting mainly comprise close to equal amounts of the haplogranite components Qz, Ab and Or, with H2O. Many felsic granites were formed by partial melting under such conditions and are low-temperature types, with crystals of zircon and other restite minerals present in the initial magma. Such magmas evolve in composition, at least initially, through fractionation of that restite. If one of the four haplogranite components either becomes depleted or too low in amount to contribute further to the melt, then melting may proceed to higher temperatures without a contribution from that component. Melting will advance to significantly higher temperatures if there is a critical deficiency in one or more components and a high-temperature granite magma forms, in which zircon is completely soluble. Such magmas are extracted from the source in a completely molten state and may evolve by fractional crystallisation. They are monzonitic, tonalitic or A-type, depending on whether the critical deficiency occurred in the Qz, Or or H2O component. If the Ab component is critically deficient, as in pelitic rocks, the rocks may be infertile for granite production. The control that source rock compositions exert on both the physical and chemical properties of granite magmas provides a unifying element in granite genesis.

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GSA Special Papers

The Fifth Hutton Symposium on the Origin of Granites and Related Rocks

S. Ishihara
S. Ishihara
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W.E. Stephens
W.E. Stephens
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S.L. Harley
S.L. Harley
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M. Arima
M. Arima
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T. Nakajima
T. Nakajima
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Geological Society of America
Volume
389
ISBN print:
9780813723891
Publication date:
January 01, 2004

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